1. Field of the Invention
The present invention relates to a die assembly for extruding metals, such as aluminum, to produce hollow articles.
2. Related Art Statement
The so-called port-hole dies have widely been used to make aluminum hollow articles, such as a cylindrical pipe.
The port-hole die is an assembly of a male die 101 combined with a female die 102, as shown in FIGS. 23A and 23B. The male die 101 comprises an annular base 103, bridges 104 and a core 105. This core 105, supported by the bridges 104 integral therewith, is located axially of, and protrudes forwardly of, the annular base 103. The female die 102 consists of an annular base 110 and a forming hole 106 extending axially thereof.
The male die 101 has a recessed or protruding periphery 108, and the female die 102 has a protruding or recessed periphery 107 facing the periphery 108 of the male die. Those peripheries fit one in another to align the male die with the female die. Thus, a forming slit 109 is provided between the core 105 and the forming hole 106 aligned therewith.
In such a prior art port-hole die, the annular base 103 and the bridges 104 are integral with the core 105 of the male die 101. Therefore, the male die has to be replaced in its entirety with a new one, when a bearing tip 105a of the core 105 has become worn out after a long use in the extrusion process. The replacement of the male die in its entirety has raised the maintenance cost of the die assembly.
It may be possible to use a core supported by the bridges but separable from the remaining parts of the male die. In this case, a core holder may be formed integral with the bridges so that the core can solely be removed from the male die when the bearing tip of said core is abraded. The parts, except for the core, can thus be reused to remarkably lower the maintenance cost of die change.
However, in case of using the core supported by and separable from the core holder, it will be difficult for the core to be precisely placed in the holder. Thus, accurate alignment of the separable core with the forming hole will not be ensured, thereby causing a problem that uneven wall thickness, which impairs the dimensional precision of extruded hollow articles, can result.
The prior art port-hole die shown in FIGS. 23A and 23B suffers another problem resulting from its structure in which the position of core 105 relative to the forming hole 106 is regulated by the annular recess 108 mating with the annular protrusion 107. Such recesses and protrusions are formed along the peripheries of the bases 103 and 110, and are thus remote from the forming slit 109. An uneven deformation is likely to occur in the male or female die, due to the extruding pressure imparted to the rear face of the die assembly. Such an uneven deformation will disturb the alignment of said core 105 with the hole 106, whereby the wall thickness of each extruded article would become uneven in the circumferential direction.
It is important, at first, to surely protect the separable core from slipping off. The core must easily be mounted in the die assembly, without needing much labor. Further, the die assembly, including the separable core, should be manufactured easily and inexpensively. However, the previously proposed structures for holding the separable core do not necessarily meet all of these requirements.